1. Field of the Invention
The present invention relates to systems using electromagnetic transponders, that is, transceivers (generally mobile) capable of being interrogated in a contactless and wireless manner by a unit (generally fixed), called a read and/or write terminal. Generally, transponders extract the power supply required by the electronic circuits included therein from the high frequency field radiated by an antenna of the read and write terminal.
The present invention more specifically relates to systems in which several transponders are likely to receive, at the same time, the field radiated by a same read terminal. This regards, in particular, transponder readers provided with no means for isolating a transponder, for example, by introducing said transponder into a slot or the like.
2. Discussion of the Related Art
In such systems, the reader must be able to exhaustively determine the number of transponders present in its field as well as, according to the applications, the number of transponders with which it must simultaneously communicate.
FIG. 1 very schematically shows a conventional example of a data exchange system between a read/write terminal 1 and a transponder 10 of the type to which the present invention relates.
Generally, terminal 1 is essentially formed of a series oscillating circuit formed of an inductance L1 in series with a capacitor C1 and a resistor R1. This series oscillating circuit is controlled by a device 2 that may include, among other things, an amplifier or antenna coupler, a circuit for controlling and exploiting the received data especially provided with a modulator/demodulator and with a microprocessor for processing the control signals and the data. Circuit 2 generally communicates with different input/output circuits (keyboard, screen, means of transmission to a server, etc.) and/or processing circuits, not shown. The circuits of the read/write terminal draw the power required by their operation from a supply circuit (not shown) connected, for example, to the electric supply system.
A transponder 10, intended for cooperating with a terminal 1, essentially includes a parallel oscillating circuit formed of an inductance L2, in parallel with a capacitor C2 between two input terminals 11, 12 of a control and processing circuit 13. Terminals 11, 12 are in practice connected to the input of a rectifying means (not shown), the outputs of which form D.C. supply terminals of the circuits internal to the transponder. These circuits generally include, essentially, a microprocessor, a memory, a demodulator of the signals that may be received from terminal 1, and a modulator for transmitting the data to the terminal.
The oscillating circuits of the terminal and of the transponder are generally tuned on a same frequency corresponding to the frequency of an excitation signal of the terminal's oscillating circuit. This high-frequency signal (for example, at 13.56 MHz) is not only used as a transmission carrier but also as a remote supply carrier for the transponders located in the terminal's field. When a transponder 10 is located in the field of a terminal 1, a high-frequency voltage is generated across terminals 11 and 12 of its resonant circuit. This voltage, after being rectified and possibly clipped, provides the supply voltage of electronic circuits 13 of the transponder.
The high-frequency carrier transmitted by the terminal is generally amplitude-modulated by said terminal according to different coding techniques to transmit data and/or control signals to one or several transponders in the field. In return, the data transmission from the transponder to a terminal is generally performed by modulating the load formed by resonant circuit L2, C2. This load variation occurs at the rate of a sub-carrier having a frequency (for example, 847.5 kHz) smaller than that of the carrier. This load variation can then be detected by the terminal in the form of an amplitude variation or of a phase variation by means, for example, of a measurement of the voltage across capacitor C1 or of the current in the oscillating circuit.
When idle, that is, when no transponder is present in its field, a terminal 1 periodically transmits a modulated data message on the high-frequency signal. This message is a request message intended for possible transponders. This request or general call, is part of a process needed for the initialization of a communication between a transponder and a terminal.
A difficulty in establishing a communication towards one or several transponders is due to the fact that several electromagnetic transponders can simultaneously be present in the terminal's field. The latter must thus be capable of determining not only the number of transponders present in its field, but also those of the transponders that correspond to the application for which it is intended and with which it must communicate.
This constraint requires a loop operation of the terminal control program until all the transponders present in its field have been properly identified.
FIG. 2 very schematically shows an initialization flowchart of one or several communications by a read/write terminal of the type to which the present invention applies.
As soon as it is powered on and in operation, a transponder read/write terminal 1 begins (block 20, ST), after a starting, set, and test phase, a stand-by procedure during which it waits for a communication with at least one transponder to be established. This procedure essentially consists of periodically sending (block 21) a request sequence (REQ) to the possible transponder(s) present in the terminal's field. After each sending of an interrogation request 21, the reader monitors (block 22) the reception by its demodulator of an acknowledgement message (ATQ) coming from a transponder having entered its field. In the absence of an acknowledgement, the reader loops on the sending of a request 21. When it receives an acknowledgement ATQ, it then switches to a mode of checking whether the transponder really is a transponder intended for it, as well as to a possible anti-collision mode (block 23) to individualize several transponders in the field. Indeed, as a response to an interrogation request by a terminal, if several transponders are present in the field thereof, they may respond at the same time or with a sufficiently low time interval to make the result of the demodulation by the reader unexploitable. Said reader must then either select a transponder with which it wishes to communicate, or assign different channels to the different transponders.
A communication only starts when the initialization and anti-collision process illustrated in FIG. 2 is over (block 25, E), that is, when the reader has detected (block 24) that it has identified all the transponders present in its field. As long as all transponders have not been identified, it starts the sending of interrogation requests. If a transponder has been properly identified, it is placed in a state where it no longer acknowledges the interrogation requests to avoid interfering with the detection of the other possible transponders.
An initialization and anti-collision process of the type briefly described hereabove in relation with FIG. 2 is known. Illustrations of conventional methods are for example to be found in French patent applications No. 2,760,280 and 2,773,627, which are incorporated herein by reference.
The implementation of the method illustrated in FIG. 2 is most often performed by determining a maximum number of cards likely to be present in the reader's field. As described, in particular, in French patent application No2,760,280, this number can be modified by the reader according to the results of exploitation of the anti-collision process (block 23) to increase the detection probabilities and reduce the duration of the initialization process.
Indeed, the duration preceding the establishing of a communication between a read/write terminal and one or several transponders is a critical parameter in the use of such transponder systems. A transponder is often formed by a badge or by a contactless card handled by a user. If said user does not obtain an almost immediate communication with the reader, he will have a tendency to modify the position of his card or to believe that the system does not operate. It is considered that beyond a period of 100 milliseconds, the duration of establishment of a communication for a reliable operation with a transponder is too high.
Now, as discussed hereabove, this duration depends on the number of recognition loops to be performed by the reader before the communication is established to determine the number of transponders present in its field. This number of loops essentially depends on the number of transponders to be isolated.
Up to now, this number can only be determined by implementing statistic computations and probability algorithms tending to minimize the number of loops along the transponder detection.